The TOPEX/Poseidon spacecraft is scheduled for launch in August 1992 aboard an Ariane 42P launch vehicle. The mission is a collaborative scientific venture between NASA and CNES. The primary objective is to perform precise measurements of the ocean surface topography by radar altimetry from a precision, circular orbit. The satellite consists of two major sections, the Multimission Modular Spacecraft Standard Bus and the Instrument Module. The IM accommodates seven instruments, the solar array, three communication antennae, and a variety of satellite support equipment.Thermal vacuum/thermal balance testing of the TOPEX/Poseidon was performed at the Goddard Space Flight Center between 24 March 1992 and 22 April 1992. Simulation of the space environment was accomplished in a vacuum chamber using IR heating/cooling plates to establish boundary temperatures. Boundary temperatures (i.e. equivalent sink temperatures) were determined using the respective worst case, orbital average, environmental fluxes. This approach translated the absorbed fluxes for a surface into a shroud temperature which provided an equivalent amount of energy to that surface in the IR wavelength.Test results show that (1) the MMS and IM thermal designs successfully maintain all S/C components within allowable flight temperature limits during worst case cold and hot environmental conditions, (2) the IM requires no heater power during the worst case cold operational condition, and (3) the S/C thermostat/heater circuits are operational. In addition, the S/C-test thermal model has been correlated to within 5°C of the actual test temperatures.This paper describes the test thermal objectives, configuration, results, TMM correlation, and conclusions.